Members of the 3rd Generation Partnership Project (3GPP) have agreed to define specifications for what is being called “NB-IoT,” which refers to a “narrowband Internet of things.” These standards will support wireless communications for low-power equipment that may rely on batteries and that will typically send and receive only small amounts of information. Example applications for wireless devices that support NB-IoT include providing parking meters, industrials sensors, and the like with wireless communication capabilities.
The radio interface for NB-IoT will be designed so that the technology can readily be deployed by operators in portions of their existing Long Term Evolution (LTE) spectrum. Thus, it is expected that certain aspects of the NB-IoT will be defined to make the most possible use of existing LTE hardware, designs, and procedures. However, changes to the LTE specifications are likely to be made at all levels of the specifications, to reduce power consumption, improve coverage, and otherwise provide for improved operation of low-power wireless equipment.
One aspect of the existing LTE specifications is random access. In LTE, as in most communication systems, a mobile terminal may need to contact the network, via the eNodeB (3GPP terminology for an LTE base station), without yet having a dedicated resource in the uplink (from user equipment, UE, to base station). To handle this, a random access procedure is available, whereby a UE that does not have a dedicated uplink resource may transmit a signal to the base station. In the process defined by the 3GPP specifications for LTE, the first message (MSG1 or preamble) of this procedure is transmitted on a special resource reserved for random access, a physical random access channel (PRACH). This channel is limited in time and frequency, as shown in FIG. 1. The resources available for PRACH transmissions are identified to mobile terminals as part of the broadcasted system information or as part of dedicated Radio Resource Control (RRC) signaling in some cases, such as in the case of a handover.
In LTE, the random access procedure is used for a number of different reasons. Among these reasons are:                initial access, for UEs in the LTE_IDLE or LTE_DETACHED states;        an incoming handover;        resynchronization of the uplink;        a scheduling request, for a UE that is not allocated any other resource for contacting the base station; and        positioning.        
To preserve orthogonality among different user equipments (UEs—3GPP terminology for radio access terminals, including cellular telephones and machine-to-machine radio devices) in an orthogonal frequency-division multiple-access (OFDMA) or single-carrier frequency-division multiple-access (SC-FDMA) system, the time of arrival of each UE signal needs to be within the cyclic prefix (CP) of the OFDM or SC-FDMA signal. It will be appreciated that the term cyclic prefix in background art refers to the prefixing of an OFDM symbol with a repetition of the symbol's end. The cyclic prefix acts as a guard interval, so as to eliminate inter-symbol interference from the previous symbol. It also allows the linear convolution of a channel to be modelled as circular convolution, which can be performed in the frequency domain with a discrete Fourier transform. This frequency-domain processing simplifies demodulation processes in an LTE receiver.
LTE random access can be either contention-based or contention-free. The contention-based random access procedure consists of four steps, as illustrated in FIG. 2. Note that only the first step involves physical-layer processing specifically designed for random access, while the remaining three steps follow the same physical-layer processing used in uplink and downlink data transmission. The eNodeB can order the UE, through a Physical Downlink Control Channel (PDCCH), to perform a contention based random access. The UE starts the random access procedure by randomly selecting one of the preambles available for contention-based random access. The UE then transmits the selected random access preamble on the PRACH to the eNodeB in the Radio Access Network (RAN), shown in FIG. 2 as step 1.
The RAN acknowledges any preamble it detects by transmitting a random access response, which includes an initial grant to be used on the uplink shared channel, a temporary Cell Radio Network Temporary Identification (C-RNTI) for the UE, and a time alignment (TA) update. The TA update is based on the timing offset of the preamble measured by the eNodeB on the PRACH. The random access response is transmitted in the downlink to the UE (step 2) and its corresponding PDCCH message cyclic redundancy code (CRC) is scrambled with a Random Access Radio Network Temporary Identifier (RA-RNTI).
After receiving the random access response, the UE uses the grant to transmit a message back to the RAN (step 3). This message is used, in part, to trigger the establishment of RRC and in part to uniquely identify the UE on the common channels of the cell. The timing advance command that was provided to the UE in the random access response is applied in the UL transmission in message transmitted back to the RAN. The eNodeB can change the resources blocks that are assigned for transmission of this message of step 3 by sending a UL grant having its CRC scrambled with a Temporary Cell Radio Network Temporary Identifier (TC-RNTI).
The procedure ends with the RAN solving any preamble contention that may have occurred for the case that multiple UEs transmitted the same preamble at the same time. This can occur when each UE randomly selects when to transmit and which preamble to use. If multiple UEs select the same preamble for the transmission at the same time on the Random Access Channel (RACH), there will be contention between these UEs. The RAN resolves this contention using the contention resolution message, seen as step 4 in FIG. 2. This message, which is sent by the eNodeB for contention resolution, has its PDCCH CRC scrambled with the C-RNTI if the UE previously has a C-RNTI assigned. If the UE does not have a C-RNTI previously assigned has its PDCCH CRC is scrambled with the TC-RNTI.
A scenario where contention occurs is illustrated in FIG. 3, where two UEs transmit the same preamble, p5, at the same time. A third UE also transmits a random access preamble at the same time, but since it transmits with a different preamble, pi, there is no contention between this UE and the other two UEs.
For contention-free random access, the UE uses reserved preambles assigned by the base station. In this case, contention resolution is not needed, and thus only steps 1 and 2 of FIG. 2 are required. A non-contention-based random access or contention-free random access can be initiated by the eNodeB, for example, to get the UE to achieve synchronization in the uplink. The eNodeB initiates a non-contention-based random access either by sending a PDCCH order or indicating it in an RRC message. The latter of these two approaches is used in the case of a handover.
The procedure for the UE to perform contention-free random access is illustrated in FIG. 4. As with the contention-based random access, the random access response is transmitted in the downlink to the UE and its corresponding PDCCH message CRC is scrambled with the RA-RNTI. The UE considers the contention resolution successfully completed after it has received the random access response successfully. For the contention-free random access, as for the contention-based random access, the random access response contains a timing alignment value. This enables the eNodeB to set the initial/updated timing according to the UEs transmitted preamble.
Efforts currently underway with respect to the so-called Networked Society and Internet of Things (IoT) are associated with new requirements on cellular networks, e.g., with respect to device cost, battery lifetime and coverage. To drive down device and module cost for the small wireless devices that are expected to become ubiquitous, using a system-on-a-chip (SoC) solution with integrated power amplifier (PA) is highly desirable. However, it is currently feasible for state-of-the-art PA technology to allow only about 20-23 dBm transmit power when the power amplified is integrated to the SoC. This constraint on output power from the SoC solution limits uplink coverage, which is related to how much the path loss is allowed between the user terminal and base station.
Further, to maximize the coverage achievable by an integrated PA, it is necessary to reduce PA backoff. PA backoff is needed when the communication signal has a non-unity peak-to-average power ratio (PAPR), i.e., when the communication signal is not a constant envelope signal. To avoid spurious signals and out-of-band emissions from the PA when amplifying a non-constant-envelope signal, the PA must be operated at or near its linear operating region, i.e., it must be “backed off” from its high-efficiency, nonlinear operating region. The higher the PAPR is, the higher the PA backoff required. Because higher PA backoff gives rise to lower PA efficiency, it lowers device battery life time. Thus, for wireless IoT technologies, designing an uplink communication signal that has as low PAPR as possible is critically important for achieving the performance objectives for IoT devices with respect to device cost, battery lifetime and coverage.